Thirty-five illegal Mexican immigrants, part of a larger group discovered in a produce truck a week ago by police, are expected to remain in jail for at least another week, although federal authorities have said they will not be charged with a crime.

They are being held as witnesses to a felony, their own transportation into the country, and under provisions of a 1971 decision by the 9th U.S. Circuit Court of Appeals, may be kept in federal custody until attorneys for the men charged with transporting them have interviewed them.

Police spotted the van Aug. 3 in the parking lot of a Long Beach restaurant. Hearing a commotion, they had the driver open the back door, and found 53 illegal aliens, ranging in age from 4 to 53. Included in the group were 12 juveniles and a woman nine months pregnant.

Two San Diego-area men, Mark R. Rudy and Ronald Scruggs, were arrested and charged with transporting the illegal aliens, who were held as material witnesses.

By last Friday, some of the youngsters had been moved from a juvenile detention center to halfway houses, and some of the adults had been moved from county jails to community centers. One man was released into the custody of his brother, a Los Angeles resident.

All 53 will eventually be deported to Mexico by the Immigration and Naturalization Service. But for the indefinite future they will remain in Los Angeles, most of average time illegal aliens are detained as material witnesses in smuggling and transportation cases is two to three weeks. However, it is not uncommon for them to be held without charge as witnesses for up to three months.

In 1978, more than 8,000 illegal aliens were detained as witnesses in Los Angeles, San Diego and Phoenix. While most remained in jail, those accused of smuggling or transporting them often were released on bond.

"In a sense, the aliens' rights are being violated," said Gary Scales, assistant U.S. attorney in Phoenix.

"They are guilty of a federal offense for being in the country. In return for no prosecution, we wish them to be our witnesses, but I really can't justify their continued confinement because of that."

Rudolph A. Diaz, chief deputy in the federal public defender's office in Los Angeles, which represents aliens being held as material witnesses, is critical of the U.S. attorney's office for not releasing many illegal immigrants early in the proceedings.

"We have to constantly fight the government," Diaz said. "They insist they need 30 or 40 witnesses, which is nonsense. No court would hear that many witnesses."

However, federal prosecutors argue that they are hamstrung by the 1971 appeals court decision, United States vs. Mednez-Rodriguez, which requires that every illegal alien apprehended after being smuggled, transported or harbored must be held in federal custody until the defendant's attorney has a chance question him.

"Aside from the lack of investigative and prosecutive resources, the most serious obstacle to the effective enforcement of our immigration laws is the decision," wrote Michael H. Walsh, U.S. attorney in San Diego, in a report released in March.

"Incarceration obviously results in severe hardship for the witnesses," said Walsh, noting that the "beneficiary of the the defendant who knows that delay works to pressure the prosecution to dismiss, reduce or compromise the charges brought against the defendant" in the interest of incarcerated material witnesses.

Material witnesses held in Los Angeles are kept in the county jail and separated from the rest of the inmate population.

In San Diego they are housed in the federal Metropolitan Correctional Center, according to John J. Cleary, executive director of Federal Defenders of San Diego Inc... At any one time, 20 to 30 percent of the prisoners at the center are immigrant witnesses.

"The use of maximum-security facilities - treatiny these people the same as our bank robbers - is totally inappropriate," said Cleary.

A public defenders group in Los Angeles said prosecutors and judges are cooperating to speed up the processing of illegal immigrant-related cases in an attempt to reduce the amount of time witnesses spend in custody.

Lawyers for the Mexicans found in the produce truck in Long Beach Aug. 3 would not allow their clients to be interviewed. But they said illegal immigrants usually pay $200 each to be walked by guides across the border, then driven to Los Angeles, often changing vehicles a number of times.

Investigators from the Los Angeles INS office were in San Diego last week following leads that the truck that transported the 53 illegal immigrants was registered to a San Diego-area produce company.

"This is not a small operation," noted Assistant U.S. Attorney Brian D. Brad at a bail hearing last week for one of the defendants. "There's a lot of money involved and there may be a larger operation behind this defendant."

INS officials estimated that the smuggling of illegal aliens into California and Arizona grosses in excess of $50 million a year.